MEM variable capacitors are expected to be particularly suitable in microwave and millimetre wave applications such as, for example, tunable filters and voltage controlled oscillators where a high quality factor (Q) and a wide tuning range are desirable.
A MEM variable capacitor comprises a pair of electrodes: a fixed electrode that is mounted on a substrate and a movable electrode that is suspended over the fixed electrode to define an air gap between facing planar surfaces of the electrodes. When a DC control voltage is applied across the electrodes the moveable electrode moves toward the fixed electrode under electrostatic attraction. This reduces the air gap between the electrodes and increases the capacitance. The motion of the moveable electrode is restricted by a mechanical spring force. The spring force is directly proportional to the distance travelled by the moveable electrode (i.e. the reduction in the original air gap) whereas the electrostatic attractive force has a non-linear relationship with the air gap.
It can be shown mathematically that when the air gap is less than two-thirds of the initial air gap distance the electrostatic attractive force exceeds the spring force and the electrodes are pulled together. The control voltage at which this occurs is known as the “pull-in” voltage. This limits the capacitance tuning ratio to 1.5:1 which is inadequate for many applications.
Even to achieve a tuning ratio of 1.5:1 requires the operation of the device at voltages close to the pull-in voltage and therefore it is often the case that pull-in occurs. When this happens the facing planar surfaces come into contact with each other and often adhere together after the applied voltage has been removed as a result of stiction. This results in device failure. Stiction between the facing planar surfaces also leads to low yields when such devices are being fabricated.
One known approach to improving the capacitance ratio of a MEM variable capacitor is disclosed in WO 01/61848. As in the device described above a moveable electrode is anchored at each end so as to be suspended above a fixed electrode but is flexible so that it deflects when the control voltage is applied. The fixed electrode is, however, split such that here is a central active electrode that combines with the movable electrode to form the variable capacitor and two outer control electrodes to which the control voltage is applied. The central electrode is located midway between the anchors where the deflection of the flexible electrode is greatest. The electrodes are arranged so that the gap between the central electrode and the flexible electrode is less than that between the control electrodes and the flexible electrode. In operation, the maximum deflection of the flexible electrode is limited by the distance between it and the control electrodes. The minimum distance between the deflected flexible electrode and the control electrodes to avoid pull-in is again two thirds of the initial gap. If the gap between the central electrode and the flexible electrode is made less than one third of that between the control electrodes and the flexible electrode then the pull-in voltage does not limit the tuning range.
The device described in WO 01/61848 does provide a wider tuning ratio range, but pull-in still occurs in some circumstances and stiction resulting in device failure is therefore still a problem during both use and manufacture.
A paper published after a conference in San Francisco 10-13 Dec. 2000 (Development of a Wide Tuning Range MEMS Tunable Capacitor for Wireless Communication Systems, Jun Zou; Chang Liu; Schutt-Aine,J; Jinghong Chen, and Sung-Mo Hung 0-7803-6441-4/00) describes the performance of a device of the same general type as that described in WO 01/61848. The paper notes that, in a test device, after pull-in had occurred the spacing between the facing surfaces could not be reduced to zero, suggesting by way of explanation “Possible reasons are surface roughness of the two plates, the existence of residual film from sacrificial layer etching, or absolute measurement calibration”.The same paper also refers to the achievable tuning range value being dependent upon “other factors, such as surface roughness and curvature”. The facing surfaces of the capacitor plates are however fabricated using a process which relies upon the deposition of layers of material by thermal evaporation which will, if performed successfully, result in smooth surfaces and thus references to “roughness” in the paper are concerned with manufacturing errors rather than a deliberate attempt to achieve a surface which is not smooth.